1. Field of the Invention
The present invention relates to a low-voltage band-gap reference voltage bias circuit and, more specifically, to a low-voltage band-gap reference voltage bias circuit that is unaffected by temperature, power supply voltage, and variation in process in semiconductor bias circuit technology and can supply a stable reference voltage at a supply voltage of 1V or lower. The present invention has been produced from the work supported by the IT R&D program of MIC (Ministry of Information and Communication)/IITA (Institute for Information Technology Advancement) [2005-S017-02, Integrated Development of UltraLow Power RF/HW/SW SoC] in Korea.
2. Discussion of Related Art
Generally, Radio-Frequency (RF) circuits, analog mixed circuits or digital circuits that are fabricated as chips require stable and precise reference bias voltages in order to perform efficient operations.
However, reference bias voltages provided in a conventional bias circuit are apt to change over time due to a variation in temperature during the operation of the bias circuit.
In order to solve the above-described problem, a band-gap reference voltage bias circuit has been employed. The band-gap bias circuit provides stable reference voltages by using a temperature characteristic of a bipolar transistor (or a diode) under the conditions of any variation of temperature.Vref=α1V1+α2V2≈α1VBE+α2ΔVBE  (Equation 1)
In Equation 1, a voltage V1 has a characteristic that is proportional to temperature, while a voltage V2 has a characteristic that is inversely proportional to temperature. In this case, when a zero-temperature coefficient obtained by selecting appropriate values such that the sum of the characteristics of the two voltages V1 and V2 satisfies an equation α1∂V1/∂T+α2∂V2/∂T=0, a reference voltage Vref is independent of any variation of temperature.
FIG. 1 is a circuit diagram of a conventional CMOS band-gap reference voltage bias circuit. A base-emitter voltage of a bipolar transistor is inversely proportional to temperature, while a base-emitter voltage difference ΔVBE between first and second bipolar transistors Q1 and Q2 having different amounts of current is proportional to temperature. Voltages (i.e. ΔVBE) applied to both ends of the first resistor R1 are amplified by the feedback amplifier AMP. In this case, a current supplied to the first resistor R1 is ΔVBE/R1. The current ΔVBE/R1 copies the characteristic of the base-emitter voltage difference ΔVBE and is mirrored to the third PMOS transistor M3.
While a mirrored current I3 flows through the second resistor R2 and the third bipolar transistor Q3 as expressed by Equation 2. Equation 2 is a numerical expression of a band-gap reference voltage that can counteract a temperature coefficient. In this case, a coefficient k having an inverse temperature slope to the base-emitter voltage VBE3 of the third bipolar transistor Q3 is controlled by using a resistance ratio R2/R1 in order to obtain exact temperature compensation.
                              V          ref                ≈                              V                          BE              ⁢                                                          ⁢              3                                +                                                    R                2                                            R                1                                      ⁢            Δ            ⁢                                                  ⁢                          V              BE                                      ≈                              V                          BE              ⁢                                                          ⁢              3                                +                                    k              ·                              V                T                                      ⁢                                                  ⁢            ln            ⁢                                                  ⁢            n                          ≈                  1.25          ⁢                                          ⁢          V                                    (                  Equation          ⁢                                          ⁢          2                )            
However, since the conventional band-gap reference voltage bias circuit has a complete temperature compensation characteristic (i.e., a zero-temperature coefficient) at about 1.25 V as expressed by Equation 2, this bias circuit cannot be applied to circuit configurations having a sub-1V supply voltage.
In the mobile communication handsets, it is most important to design small-area low-power core chips in order to ensure high portability and durability. The development of deep sub-micron CMOS technology enables the small-area low-power (or low-voltage) core chips to be manufactured. However, even if a low supply voltage is applied to meet the low-power design specification, since a conventional band-gap bias circuit requires an operating voltage of at least 1.5 V or higher, it is difficult to design a small-area and low-power chip using the conventional band-gap bias circuit.